Ultrasonic welding allows accurate and precise application of energy to selectively melt or weld the desired portions of a plastic assembly. Ultrasonic assembly is suitable for most thermoplastic materials, and is widely used to weld thermoplastic parts in the automotive, packaging, electronic, and consumer industries. In practice, high-frequency (ultrasonic) mechanical vibrations are transmitted by the ultrasonic welding machine to mating plastic parts. At the joint or interface of the two parts, a combination of applied force and surface and/or intermolecular friction increases the temperature until the melting point of the thermoplastic is reached. The ultrasonic energy is then removed and a molecular bond or weld is produced between the two plastic parts.
An ultrasonic welding system typically contains a high-frequency power supply (usually 20-40 kHz). The high-frequency energy is directed into a horn Which is a bar or a metal section, typically of titanium, aluminum, or hardened steel, dimensioned to be resident at the applied frequency. The horn contacts the workpiece and transmits the mechanical vibrations into it. A fixture or nest supports and aligns the two parts to be welded. It is generally made of aluminum or steel, and is sometimes lined with cast urethane or another material that is resilient.
Proper joint design is essential for optimum welding results. Factors such as the type of material, part geometry, and requirements of the welded joint must be considered with determining the design. A joint should have a small, uniform initial contact area and some means of alignment to concentrate the ultrasonic energy for rapid localized energy dissipation. An energy director, the most commonly used design, consists of a small triangular bead of material on the part surface to be welded. In one type of weld, a sheer, or interference joint is used to obtain a high-strength hermetic seal. During welding, the interfaces melt and telescope together, producing a weld in the shear mode.
Many types of thermoplastic polymers can be welded. Amorphous resins such as polystyrene, acrylonitrile-butadiene-styrene (ABS), polycarbonate, etc., are very energy efficient and are generally preferred for ultrasonic welding. They are characterized by a random molecular arrangement and a broad softening temperature range. This allow the material to flow easily without premature solidification. Resins that have higher levels of crystallinity require higher ultrasonic energy levels because of their highly-ordered molecular structure. Some dissimilar resins can be welded if their glass transition temperatures are similar (typically within about 40.degree. F.). And, they have somewhat similar molecular structures. Typical examples are ABS-to-acrylic, polycarbonate-to-acrylic, and polystyrene-to-polyphenyleneoxide.
Many other factors, such as moisture content, fillers, and mold release agents, affect the ability to ultrasonically weld thermoplastic materials. Optimum welding typically requires that the plastic parts be dried prior to welding. Fillers are known to inhibit ultrasonic welding when levels of filling exceed 30-35%. Above these levels, strong welds cannot be assured because the joint areas contain insufficient fusible material. Also, fillers such as glass and minerals, can be abrasive and cause excessive horn wear at levels above 35%. Mold release agents lower the material's coefficient of friction and adversely affect the weld. Weld surfaces should preferably be cleaned with suitable cleaners to remove any surface contaminants prior to welding.
The conventional wisdom in the art of ultrasonic welding has always indicated that the joint to be welded should be thoroughly cleaned and be free of foreign material. For this reason, it has been difficult and expensive to weld plastic parts that have metal plating on them. Metal plating in the area of the weld joint must be removed in order to assure a proper weld. This requires that plated plastic parts used, for example, as shielded RF shielded housings, must have the platings selectively masked off or removed prior to welding. Clearly it would be an advantageous improvement in the state of the art to be able to weld fully plated plastic parts.